fisheries report brief final
Evaluating the Effectiveness of Fish Stock
Rebuilding Plans in the United States
Fish stock rebuilding plans have proven successful in reducing ishing pressure on many
overished stocks and stock sizes have generally increased. However, in some cases isheries
have not rebuilt as quickly as the plans projected, due to factors such as overestimation of
the size of stocks and incidental catch by isheries targeting other species. Even when ishing
is reduced appropriately for the actual stock size, the rate at which rebuilding occurs will
depend on ecological and other environmental conditions. Because of all these factors, it is
dificult to make accurate predictions of the time it will take stocks to rebuild. Therefore,
rebuilding plans that focus more on meeting selected ishing mortality targets than on
adhering to strict schedules for achieving rebuilding may be more robust to assessment
uncertainties, environmental variability, and the effect of ecological interactions.
ell-managed,
the Sustainable
productive
Fisheries Act, more
isheries
clearly deined overprovide a livelihood, a
ishing and required
nutritious source of food,
rebuilding of overished
and recreational activity
stocks within a specifor coastal communities
ied time limit. In
around the country.
2006, Congress made
However, if overishing
additional mandates
takes place and stocks
for conserving and
experience serious
rebuilding ish stocks
Figure 1. A double-rigged shrimp trawler.
declines, the lost producand strengthening the
Credit: NOAA
tivity affects the ishing
role of scientiic
industry and communities that depend on
advice in isheries management. The Act now
ishing, and raises concerns about the overall
includes regulatory provisions such as ending
health of the associated marine ecosystem. In
overishing immediately, annual catch limits
the United States, the 1976 Magnuson-Stevens
and accountability measures.
Fishery Conservation and Management Act
To meet these provisions, rebuilding plans
was the irst piece of major legislation to
have required substantial reductions in catch
regulate federal isheries. Although the Act
for many isheries, resulting in social and
contained language to “prevent overishing,”
economic impacts to ishing communities and
it focused on developing the domestic ishing
industry. This report reviews the technical
industry. Major declines in the productivity of
speciications that underlie current federallyseveral important isheries led Congress to
implemented rebuilding plans, and the
amend the Act 1996. The amendment, called
outcomes of those plans.
W
A ishery is a group of ishing operations targeting
similar species, using similar gear, during a similar
time of year and/or within the same area.
A stock is a single species of ish in one
geographical area.
Overishing occurs when the rate of ishing
exceeds FMSY.
A stock is considered overished when the
population is driven below the maximum
sustainable yield-derived reference point for that
stock. For many stocks, this reference point is set
at 50 percent of BMSY.
The Challenges of Rebuilding Plans
Rebuilding plans based on MSY have proven
successful for many stocks. The committee
considered a subset of 55 stocks that were put
under rebuilding plans at some point between 1997
and 2012. Of these, 10 have rebuilt, 5 are rebuilding,
and 11 of the stocks are on the way to rebuilding if
recent decreases in ishing mortality are sustained.
However, data from the most recent stock assessments indicate that 20 of the 55 stocks analyzed were
not actually overished at the time they were placed in
rebuilding plans, a consequence of uncertainties in
estimating ish stock abundance. Such uncertainties
cut both ways: retrospective assessments indicate that
stock size may also be overestimated, such that some
stocks classiied as healthy were actually overished.
The remaining 9 of the 55 stocks are still subject to
overishing even though very strict ishing limits have
been set (below 75% FMSY). Those failures can relect
problems with implementing the rebuilding plans,
dificulties in reducing unintentional harvest of the
stock (bycatch), or uncertainties in estimating stock
size.
MSY Reference Points
A major challenge comes from the fact that current
rebuilding plans use a static metric of MSY, which in
theory represents average conditions. In reality,
ecosystems are dynamic and as a consequence MSY
varies with factors such as changes in environmental
1.5
0.5
1.0
Increasing F
0.0
Fishing mortality FMSY
The National Marine Fishery Service evaluates
the status of US ish stocks to determine which
stocks are overished; that is, which stocks are
too small in size to sustain continued productive
isheries. Once a stock is classiied as overished,
the appropriate Regional Fishery Management
Council selects and implements a rebuilding
plan.
Rebuilding plans are simple in theory: annual
catch limits are set to reduce ishing, which
allows the stock to grow and recover. However,
in order to design a rebuilding plan, ishery
managers need to anticipate how the stock may
respond to different levels of ishing pressure.
Currently, rebuilding plans use a concept called
Maximum Sustainable Yield (MSY) to determine when a stock is overished, and to set
annual catch limits and rebuilding targets for
stock size (see Box 1).
2.0
How Rebuilding Plans Work
0.0
0.5
1.0
1.5
2.0
Biomass BMSY
Figure 2. As ishing mortality increases over time (red line), a ish
stock will go from an unished state (triangle in lower right), to
gradually becoming overished (diamond in upper left). At this point,
a rebuilding plan might go into effect, leading to reduced ishing
(green line). According to theory, this should allow the stock to
recover and biomass to increase until it reaches BMSY.
conditions and ecological interactions. Generating
reliable estimates of MSY depends on having extensive information about the biology of the species and
its abundance in the years before it was overished.
This wealth of information is only available for a
relatively few stocks, hence there is considerable
uncertainty in the MSY estimates for most stocks.
Although the MSY approach has been successful for
some isheries, management based on MSY can fall
short in addressing ecosystem complexity and variability and in accounting for uncertainty in the
estimates of reference points.
Box 1. Maximum Sustainable Yield
To be considered sustainable, a population of ish should
produce enough young to replace the ish that die each
year, due to natural causes and to ishing. The maximum,
sustainable, long-term average yield that can be taken
from a ish stock is called the Maximum Sustainable
Yield (MSY).
The population level, or biomass, that generates MSY is
called the Biomass MSY, or BMSY.
The rate of ishing, called ishing mortality, which will
maintain the average population size at BMSY is called the
ishing mortality MSY, or FMSY.
60
total overfished
newly declared overfished
rebuilding
rebuilt
undefined
currently possible to incorporate them
into models to improve projections to a
degree of accuracy that is useful for
management.
40
30
20
0
10
Number of stocks
50
Mixed Stock Fisheries
Fish do not live in isolation—each stock
is part of a community of species that
live in the same waters. For example,
when a ishing net is cast into the ocean,
it can capture several different ish
species. This is called a mixed-stock
ishery.
When one stock within a mixed-stock
ishery is declared overished, reductions in ishing required by the
rebuilding plans affect all the stocks in
the ishery, leading to a loss of yield and
income. For example, juvenile red
2000
2005
2010
snapper in the Gulf of Mexico are
Year
incidentally caught during shrimp trawl
Figure 3. Rebuilding plans have reduced overishing and helped rebuild many
ishing, driving the red snapper to
ish stocks. However, some stocks have proven slower to rebuild, and others
overished status. Devices were installed
remain overished. This chart shows the status each year of all 85 stocks
in shrimp nets to reduce bycatch, but
designated overished between 1997 and 2012.
were not suficient to end overishing.
Subsequent
rebuilding
plans included a shrimp trawl
Uncertainty in Stock Assessments
ishing threshold, in addition to ishing limits for red
Because it is not possible to count every individual
snapper.
ish in a stock, scientists rely on a variety of sampling
and statistical methods to estimate abundance. These
estimates are used in models to project trends in
future stock size. The accuracy of the population
estimates and the projections depends on the quality
and amount of data available and the ability of
models to reproduce the primary determinants of the
species’ population growth. The frequency of stock
assessments can vary widely, both within and among
regions, from stocks that are assessed annually to
stocks that have never been assessed. As more data
are collected and new models and assessment
methods become available, past estimates of the
status of ish stocks can change substantially. For
example, some stocks that were previously classiied
as overished (and put under rebuilding plans) would
not have been considered overished based on the
most recent assessments. The inverse may also have
occurred, with overished stocks misclassiied as
not overished.
Climate, environmental conditions, and ecosystem
interactions can have signiicant effects on the rate at
which a stock rebuilds. Although these factors affect
the time it takes for a stock to rebuild, it is not
Alternative Management Strategies
Current rebuilding plans rely on a prescriptive
approach, which has had demonstrated successes in
identifying and rebuilding some ish stocks. However,
the plans’ focus on achieving rebuilding targets within
set timeframes forces reliance on forecasts and estimates of MSY-based reference points, which often
carry a high level of uncertainty. Rebuilding plans that
focus on meeting selected ishing mortality targets
may be more effective than a plan with an exact time
period for rebuilding. The report’s authoring
committee made several suggestions for alternative
management strategies for rebuilding ish stocks.
Gradual Reductions in Annual Catch
Delaying reductions in annual catch until the stock
biomass has declined to overished status means that
ishery managers must then make immediate and
substantial decreases in ishing. Alternative management actions, such as harvest control rules that
promptly but gradually reduce ishing as estimated
stock size falls would result in a lower likelihood of a
stock becoming overished.
Fishing Mortality Reference Points
Fishing mortality reference points seem to be more
robust to uncertainty than biomass reference points,
both in the context of rebuilding and more generally. Estimates of biomass are expected to change,
and hopefully improve, as stocks rebuild—but
predicting these changes is dificult as it requires
predicting average future recruitment of juvenile
ish to the adult population. In contrast, setting
reference points based on ishing mortality requires
comparatively less information.
Environmental Considerations
Most isheries select for the largest ish, and by
doing so reduce the average age of the ished population. A rebuilding strategy based on maintaining
reduced ishing mortality for an extended period—
longer than the mean generation time of the
stock—would help restore the stock’s age structure
(i.e., increase the average age) and would be more
robust to changing environmental conditions than
strategies that require rebuilding to pre-speciied
biomass targets by a given time limit.
Setting Fishing Limits Below FMSY
The Magnuson-Stevens Act speciies that stocks
designated as overished must rebuild within a
maximum time period (usually 10 years). This
strategy means rebuilding usually occurs quickly
once a stock is designated overished. However, if
rebuilding is slower on average than expected when
the plan was adopted—for example, due to unfavorable environmental conditions that affect recruitment
of juveniles—ever more severe controls on ishing
must be applied to try to keep rebuilding “on
schedule”. These reductions in ishing can have
signiicant economic and social impacts to the
ishing industry and community. Maintaining
ishing at some constant level below FMSY may help
achieve rebuilding goals on a schedule that accommodates natural variability in stock productivity.
Data-Poor Stocks
Analytical assessments are not available for many
of the stocks classiied as overished because there
are not enough data and information to build the
required models and to estimate MSY reference
points. Without these data, catch limits are dificult
to establish. In these cases, empirical rebuilding
strategies that rely on input controls to reduce
ishing—for example, reductions in ishing operations, or closing ishing areas—may be more
effective and defensible than strategies based on
annual catch limits and BMSY targets.
Socio-Economic Impacts of Rebuilding Plans
It is clear that reducing ishing to meet rebuilding
targets can have severe social and economic
impacts on ishing communities and the ishing
industry. However, socio-economic information
is not readily available to evaluate the broader
and long-term impacts of rebuilding plans.
Retrospective reviews of the socio-economic
impacts of rebuilding plans would help in reining
rebuilding plans and objectives, thus improving the
consequences of future plans.
Download free or purchase the published report at http:/www.nap.edu or call (800) 624-6242 for bulk purchases.
For more information, visit http://dels.nas.edu/osb or call (202) 334-2714.
Committee on Evaluating the Effectiveness of Stock Rebuilding Plans of the 2006 Fishery Conservation and Management
Reauthorization Act: Ana M. Parma (Co-Chair), Centro Nacional Patagonico, Chubut, Argentina; Patrick J. Sullivan
(Co-Chair), Cornell University, Ithaca; Jeremy Collie, University of Rhode Island, Narragansett; Troy W. Hartley, College of
William & Mary, Gloucester Point, Virginia; William Heyman, Texas A&M University, College Station; Robert Johnston,
Clark University, Worcester, Massachusetts; André E. Punt, University of Washington, Seattle; Kenneth A. Rose, Louisiana
State University, Baton Rouge; James Sanchirico, University of California, Davis; Michael P. Sissenwine, Woods Hole
Oceanographic Institution, Massachusetts; George Sugihara, University of California, San Diego; Kim Waddell (Study
Director), Sherrie Forrest (Associate Program Oficer), Heather Chiarello (Senior Program Assistant), Constance Karras
(Postgraduate Fellow), National Research Council.
The National Academies appointed the above committee of experts to address the speciic task requested by
the National Oceanic and Atmospheric Administration. The members volunteered their time for this activity;
their report is peer-reviewed and the inal product signed off by both the committee members and the National
Academies. This report brief was prepared by the National Research Council based on the committee’s report.
Permission granted to reproduce this brief in its entirety with no additions or alterations.
Permission for images/igures must be obtained from their original source.
© 2013 The National Academy of Sciences
Rebuilding Plans in the United States
Fish stock rebuilding plans have proven successful in reducing ishing pressure on many
overished stocks and stock sizes have generally increased. However, in some cases isheries
have not rebuilt as quickly as the plans projected, due to factors such as overestimation of
the size of stocks and incidental catch by isheries targeting other species. Even when ishing
is reduced appropriately for the actual stock size, the rate at which rebuilding occurs will
depend on ecological and other environmental conditions. Because of all these factors, it is
dificult to make accurate predictions of the time it will take stocks to rebuild. Therefore,
rebuilding plans that focus more on meeting selected ishing mortality targets than on
adhering to strict schedules for achieving rebuilding may be more robust to assessment
uncertainties, environmental variability, and the effect of ecological interactions.
ell-managed,
the Sustainable
productive
Fisheries Act, more
isheries
clearly deined overprovide a livelihood, a
ishing and required
nutritious source of food,
rebuilding of overished
and recreational activity
stocks within a specifor coastal communities
ied time limit. In
around the country.
2006, Congress made
However, if overishing
additional mandates
takes place and stocks
for conserving and
experience serious
rebuilding ish stocks
Figure 1. A double-rigged shrimp trawler.
declines, the lost producand strengthening the
Credit: NOAA
tivity affects the ishing
role of scientiic
industry and communities that depend on
advice in isheries management. The Act now
ishing, and raises concerns about the overall
includes regulatory provisions such as ending
health of the associated marine ecosystem. In
overishing immediately, annual catch limits
the United States, the 1976 Magnuson-Stevens
and accountability measures.
Fishery Conservation and Management Act
To meet these provisions, rebuilding plans
was the irst piece of major legislation to
have required substantial reductions in catch
regulate federal isheries. Although the Act
for many isheries, resulting in social and
contained language to “prevent overishing,”
economic impacts to ishing communities and
it focused on developing the domestic ishing
industry. This report reviews the technical
industry. Major declines in the productivity of
speciications that underlie current federallyseveral important isheries led Congress to
implemented rebuilding plans, and the
amend the Act 1996. The amendment, called
outcomes of those plans.
W
A ishery is a group of ishing operations targeting
similar species, using similar gear, during a similar
time of year and/or within the same area.
A stock is a single species of ish in one
geographical area.
Overishing occurs when the rate of ishing
exceeds FMSY.
A stock is considered overished when the
population is driven below the maximum
sustainable yield-derived reference point for that
stock. For many stocks, this reference point is set
at 50 percent of BMSY.
The Challenges of Rebuilding Plans
Rebuilding plans based on MSY have proven
successful for many stocks. The committee
considered a subset of 55 stocks that were put
under rebuilding plans at some point between 1997
and 2012. Of these, 10 have rebuilt, 5 are rebuilding,
and 11 of the stocks are on the way to rebuilding if
recent decreases in ishing mortality are sustained.
However, data from the most recent stock assessments indicate that 20 of the 55 stocks analyzed were
not actually overished at the time they were placed in
rebuilding plans, a consequence of uncertainties in
estimating ish stock abundance. Such uncertainties
cut both ways: retrospective assessments indicate that
stock size may also be overestimated, such that some
stocks classiied as healthy were actually overished.
The remaining 9 of the 55 stocks are still subject to
overishing even though very strict ishing limits have
been set (below 75% FMSY). Those failures can relect
problems with implementing the rebuilding plans,
dificulties in reducing unintentional harvest of the
stock (bycatch), or uncertainties in estimating stock
size.
MSY Reference Points
A major challenge comes from the fact that current
rebuilding plans use a static metric of MSY, which in
theory represents average conditions. In reality,
ecosystems are dynamic and as a consequence MSY
varies with factors such as changes in environmental
1.5
0.5
1.0
Increasing F
0.0
Fishing mortality FMSY
The National Marine Fishery Service evaluates
the status of US ish stocks to determine which
stocks are overished; that is, which stocks are
too small in size to sustain continued productive
isheries. Once a stock is classiied as overished,
the appropriate Regional Fishery Management
Council selects and implements a rebuilding
plan.
Rebuilding plans are simple in theory: annual
catch limits are set to reduce ishing, which
allows the stock to grow and recover. However,
in order to design a rebuilding plan, ishery
managers need to anticipate how the stock may
respond to different levels of ishing pressure.
Currently, rebuilding plans use a concept called
Maximum Sustainable Yield (MSY) to determine when a stock is overished, and to set
annual catch limits and rebuilding targets for
stock size (see Box 1).
2.0
How Rebuilding Plans Work
0.0
0.5
1.0
1.5
2.0
Biomass BMSY
Figure 2. As ishing mortality increases over time (red line), a ish
stock will go from an unished state (triangle in lower right), to
gradually becoming overished (diamond in upper left). At this point,
a rebuilding plan might go into effect, leading to reduced ishing
(green line). According to theory, this should allow the stock to
recover and biomass to increase until it reaches BMSY.
conditions and ecological interactions. Generating
reliable estimates of MSY depends on having extensive information about the biology of the species and
its abundance in the years before it was overished.
This wealth of information is only available for a
relatively few stocks, hence there is considerable
uncertainty in the MSY estimates for most stocks.
Although the MSY approach has been successful for
some isheries, management based on MSY can fall
short in addressing ecosystem complexity and variability and in accounting for uncertainty in the
estimates of reference points.
Box 1. Maximum Sustainable Yield
To be considered sustainable, a population of ish should
produce enough young to replace the ish that die each
year, due to natural causes and to ishing. The maximum,
sustainable, long-term average yield that can be taken
from a ish stock is called the Maximum Sustainable
Yield (MSY).
The population level, or biomass, that generates MSY is
called the Biomass MSY, or BMSY.
The rate of ishing, called ishing mortality, which will
maintain the average population size at BMSY is called the
ishing mortality MSY, or FMSY.
60
total overfished
newly declared overfished
rebuilding
rebuilt
undefined
currently possible to incorporate them
into models to improve projections to a
degree of accuracy that is useful for
management.
40
30
20
0
10
Number of stocks
50
Mixed Stock Fisheries
Fish do not live in isolation—each stock
is part of a community of species that
live in the same waters. For example,
when a ishing net is cast into the ocean,
it can capture several different ish
species. This is called a mixed-stock
ishery.
When one stock within a mixed-stock
ishery is declared overished, reductions in ishing required by the
rebuilding plans affect all the stocks in
the ishery, leading to a loss of yield and
income. For example, juvenile red
2000
2005
2010
snapper in the Gulf of Mexico are
Year
incidentally caught during shrimp trawl
Figure 3. Rebuilding plans have reduced overishing and helped rebuild many
ishing, driving the red snapper to
ish stocks. However, some stocks have proven slower to rebuild, and others
overished status. Devices were installed
remain overished. This chart shows the status each year of all 85 stocks
in shrimp nets to reduce bycatch, but
designated overished between 1997 and 2012.
were not suficient to end overishing.
Subsequent
rebuilding
plans included a shrimp trawl
Uncertainty in Stock Assessments
ishing threshold, in addition to ishing limits for red
Because it is not possible to count every individual
snapper.
ish in a stock, scientists rely on a variety of sampling
and statistical methods to estimate abundance. These
estimates are used in models to project trends in
future stock size. The accuracy of the population
estimates and the projections depends on the quality
and amount of data available and the ability of
models to reproduce the primary determinants of the
species’ population growth. The frequency of stock
assessments can vary widely, both within and among
regions, from stocks that are assessed annually to
stocks that have never been assessed. As more data
are collected and new models and assessment
methods become available, past estimates of the
status of ish stocks can change substantially. For
example, some stocks that were previously classiied
as overished (and put under rebuilding plans) would
not have been considered overished based on the
most recent assessments. The inverse may also have
occurred, with overished stocks misclassiied as
not overished.
Climate, environmental conditions, and ecosystem
interactions can have signiicant effects on the rate at
which a stock rebuilds. Although these factors affect
the time it takes for a stock to rebuild, it is not
Alternative Management Strategies
Current rebuilding plans rely on a prescriptive
approach, which has had demonstrated successes in
identifying and rebuilding some ish stocks. However,
the plans’ focus on achieving rebuilding targets within
set timeframes forces reliance on forecasts and estimates of MSY-based reference points, which often
carry a high level of uncertainty. Rebuilding plans that
focus on meeting selected ishing mortality targets
may be more effective than a plan with an exact time
period for rebuilding. The report’s authoring
committee made several suggestions for alternative
management strategies for rebuilding ish stocks.
Gradual Reductions in Annual Catch
Delaying reductions in annual catch until the stock
biomass has declined to overished status means that
ishery managers must then make immediate and
substantial decreases in ishing. Alternative management actions, such as harvest control rules that
promptly but gradually reduce ishing as estimated
stock size falls would result in a lower likelihood of a
stock becoming overished.
Fishing Mortality Reference Points
Fishing mortality reference points seem to be more
robust to uncertainty than biomass reference points,
both in the context of rebuilding and more generally. Estimates of biomass are expected to change,
and hopefully improve, as stocks rebuild—but
predicting these changes is dificult as it requires
predicting average future recruitment of juvenile
ish to the adult population. In contrast, setting
reference points based on ishing mortality requires
comparatively less information.
Environmental Considerations
Most isheries select for the largest ish, and by
doing so reduce the average age of the ished population. A rebuilding strategy based on maintaining
reduced ishing mortality for an extended period—
longer than the mean generation time of the
stock—would help restore the stock’s age structure
(i.e., increase the average age) and would be more
robust to changing environmental conditions than
strategies that require rebuilding to pre-speciied
biomass targets by a given time limit.
Setting Fishing Limits Below FMSY
The Magnuson-Stevens Act speciies that stocks
designated as overished must rebuild within a
maximum time period (usually 10 years). This
strategy means rebuilding usually occurs quickly
once a stock is designated overished. However, if
rebuilding is slower on average than expected when
the plan was adopted—for example, due to unfavorable environmental conditions that affect recruitment
of juveniles—ever more severe controls on ishing
must be applied to try to keep rebuilding “on
schedule”. These reductions in ishing can have
signiicant economic and social impacts to the
ishing industry and community. Maintaining
ishing at some constant level below FMSY may help
achieve rebuilding goals on a schedule that accommodates natural variability in stock productivity.
Data-Poor Stocks
Analytical assessments are not available for many
of the stocks classiied as overished because there
are not enough data and information to build the
required models and to estimate MSY reference
points. Without these data, catch limits are dificult
to establish. In these cases, empirical rebuilding
strategies that rely on input controls to reduce
ishing—for example, reductions in ishing operations, or closing ishing areas—may be more
effective and defensible than strategies based on
annual catch limits and BMSY targets.
Socio-Economic Impacts of Rebuilding Plans
It is clear that reducing ishing to meet rebuilding
targets can have severe social and economic
impacts on ishing communities and the ishing
industry. However, socio-economic information
is not readily available to evaluate the broader
and long-term impacts of rebuilding plans.
Retrospective reviews of the socio-economic
impacts of rebuilding plans would help in reining
rebuilding plans and objectives, thus improving the
consequences of future plans.
Download free or purchase the published report at http:/www.nap.edu or call (800) 624-6242 for bulk purchases.
For more information, visit http://dels.nas.edu/osb or call (202) 334-2714.
Committee on Evaluating the Effectiveness of Stock Rebuilding Plans of the 2006 Fishery Conservation and Management
Reauthorization Act: Ana M. Parma (Co-Chair), Centro Nacional Patagonico, Chubut, Argentina; Patrick J. Sullivan
(Co-Chair), Cornell University, Ithaca; Jeremy Collie, University of Rhode Island, Narragansett; Troy W. Hartley, College of
William & Mary, Gloucester Point, Virginia; William Heyman, Texas A&M University, College Station; Robert Johnston,
Clark University, Worcester, Massachusetts; André E. Punt, University of Washington, Seattle; Kenneth A. Rose, Louisiana
State University, Baton Rouge; James Sanchirico, University of California, Davis; Michael P. Sissenwine, Woods Hole
Oceanographic Institution, Massachusetts; George Sugihara, University of California, San Diego; Kim Waddell (Study
Director), Sherrie Forrest (Associate Program Oficer), Heather Chiarello (Senior Program Assistant), Constance Karras
(Postgraduate Fellow), National Research Council.
The National Academies appointed the above committee of experts to address the speciic task requested by
the National Oceanic and Atmospheric Administration. The members volunteered their time for this activity;
their report is peer-reviewed and the inal product signed off by both the committee members and the National
Academies. This report brief was prepared by the National Research Council based on the committee’s report.
Permission granted to reproduce this brief in its entirety with no additions or alterations.
Permission for images/igures must be obtained from their original source.
© 2013 The National Academy of Sciences